So, I was watching a program about tunnelling under rivers and more specifically small sea's etc. In it, they showed the workers spraying a rubbery concrete onto an supporting lattice. Nothing new I know, but I thought, "hey, could a similar technique be utilised in orbit?" So, here's my idea; inflate a huge balloon (any, shape you fancy), climb inside, and spray a layer of sticky foam to the inside. Maybe a layer to the outside if we're feeling brave. Perhaps if the balloon size were enough, we could just put a few layers inside. Each maybe a couple of feet thick. Allow the whole lot some time to cure, then spray a thinner tougher surface to it. Hey presto, the biginnings of a habitat. Something might have to be done about the radiation, but I'm sure that that could be easily solved by some material engineers. Openings could then simply be cut, and airlocks installed. The beauty would be the lightness of the materials used.

First off, how're you going to inflate a balloon without using materials that are already strong enough to preclude the use of this spray concrete? If you need the spray concrete, it'd explode before you finished inflating it, and if it didn't then you didn't really need the spray except for finishing touches.

Second, how're you going to climb inside? It'd have to be perfectly sealed to contain the atmosphere inside (No, I'm not talking about 78%N 21%O, any kind of gas is an atmosphere), and it'd be rather awkward to climb inside before you inflated it. So you have to cut the airlocks before you shore up the structure... It's like trying to build the Taj Mahal out of toilet paper: it doesn't work.

Thirdly, you need all the little things that go into making a space station inhabitable: electrical and computer systems, life support, radiation shielding, etc., and all those things will be wasting space if they're not inside the walls. Cubic comes at a premium, and you have to design everything for maximum efficiency. Everything that can be put inside of a wall must be, otherwise your costs will skyrocket.

Finally, how would you "cut" the airlock? Puncture the shell, it'll likely shatter (unless you put in structural support that will be absolutely worthless after you put the airlocks in). Not to mention the fact that you've just lost all your atmosphere, which cost you several million dollars to haul up there (assuming you're in earth orbit -- if you're around anything else, start counting in billions).

Yeah yeah, but imagine the end has a hole the size of an airlock (perhaps already attached to this "balloon"), and you only have to let in enough gas (whatever you like) to fully inflate the balloon. It'd be low pressure for sure, so a pressure suit would still be required until the internal sprayed foam surfaces had hardened enough for a more substantial atmosphere to be used. Astronauts would enter via the airlocked end with spraying equipment. Once that's done, further facilities can be brought in to make it more useful. The difference is you could lift all of the equipment to make this inflated bubble in one shot, and it could be big, really big. I'm thinking about a balloon the size of your typical hot air balloon. Material wouldn't have to be special because it's just a surface to spray the real working material to.

You may have to use a donut shaped ballon. If you are going to shoot a concrete spray nozzle in 0g, You'll need some sort of an anchoring system, gyrotropism would be the cheapest, and the most efficient. It'll probabably need a centralized control station and tunnels extending to the balloon. Mainly to prevent the wobble of a possible station tangeant to the balloon.

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Does anyone know if this has been rolled into the CEV program at NASA? I've noticed that they are using the inflatable idea in all of their promo animations of 'Mars Missions' to date. It seems that NASA is going to have their hands full developing the CEV. Hopefully, they'll quickly realize that they'll need more than one single ship to do all of the stuff they're claiming they want to do.

The real quest would be, does a spray on concrete structure offer more protection pound per pound than an inflatable structure that is using materials such as kevlar?

with modern materials technology i'm not too sure it would. i don't think concrete is the right choice; metallic foam has been worked at/tested (but even that seemed less promising than other materials afaik)

for habitats on the lunar/mars surface many proposals suggest a covering of soil or rock chippings to provide increased protection from radiation, as well as locating the habitat within hollow lava tubes

with modern materials technology i'm not too sure it would. i don't think concrete is the right choice; metallic foam has been worked at/tested (but even that seemed less promising than other materials afaik)

for habitats on the lunar/mars surface many proposals suggest a covering of soil or rock chippings to provide increased protection from radiation, as well as locating the habitat within hollow lava tubes

I would tend to agree myself, especially with the possibilities of carbon nano tubes looming on the horizon. I think multiple layers of various materials have been proven to be much better sheilding than concrete when it comes to micro meteorites and orbital debris. Concrete is great for some things but I don't see it getting used much for orbital habitats. Lunar and martian surface though would be a different story, especially if the concrete is made there using local materials.